This disclosure concerns aircraft propulsion systems, and more particularly, aircraft propulsion systems in which a propulsive fan is driven by electrical energy derived from an aircraft combustion engine.
Vapour trails are artificial clouds that are visible trails of condensed water vapour caused by combustion products exhausted from aircraft engines. They may be formed as warm, moist exhaust gas mixes with ambient air, and arise from the precipitation of microscopic water droplets or, if the air is cold enough, tiny ice crystals. The term “vapour trails” is intended to refer both to condensation trails (that is to say “contrails”) from aircraft and to water and/or ice precipitation in or attributable to the exhaust plumes from engines.
It is understood that, depending on the timescale considered, the climate-warming impact of aircraft exhaust vapour trails and resulting vapour trail-cirrus may be of a magnitude similar to, or perhaps even greater than, that of the CO2 emitted by aircraft, and therefore may represent a significant element of aviation's total climate impact. Various attempts at vapour trail reduction or elimination have been proposed in the prior art.
US2010/0122519 describes the use of ultra-low sulphur aviation fuel as an alternative to conventional fuel to reduce sulphur by-product generation and hence reduce contrail formation.
US2008/072577 discloses suppression of vapour trail formation by reducing exhaust water vapour content through use of a heat exchanger and condenser arrangement. However such equipment potentially introduces significant weight into the engine. Furthermore, the weight penalty is incurred throughout the full duration of a flight, even though vapour trail suppression may only be required for a small percentage of the flight time.
US2010/132330 proposes suppression of vapour trail formation through the use of directed electromagnetic energy into the engine exhaust plume. However the additional energy required to operate the system could represent a significant portion of the engine power and thus incur a fuel-burn penalty. Further, in military applications, the emission of powerful electromagnetic radiation has the undesirable effect of increasing the aircraft's detectability.
Other examples in the prior art suggest attempted modification or suppression of vapour trails through the use of chemicals injected either into the engine or into the exhaust plume. Such solutions present the prospect of additional pollution, incur a weight penalty through the need to carry fuel additives with potentially little or no calorific value of their own, and may present challenges to engine reliability and/or component life.
The strategy of avoiding regions prone to vapour trail formation and/or persistence through the routing of aircraft around, above and/or below such regions has the disadvantage that it increases workload for air traffic control and/or pilots, reduces airspace capacity and, in the case of routing around regions prone to vapour trail formation or persistence (which can be tens or hundreds of kilometers in horizontal extent), the length of the route followed by the aircraft is increased, resulting in a fuel-burn penalty. Additionally in the case of climbing so as to fly above regions prone to vapour trail formation or persistence, additional fuel is burned to provide the increased thrust necessary to perform the climb. If aircraft are scheduled to fly below regions prone to vapour trail formation or persistence, additional fuel may be burned subsequently if the aircraft is to return to its optimal cruising altitude once the aircraft has passed the avoided region.
Prior art proposals for vapour trail mitigation generally assume the use of a traditional gas turbine engine configuration, such as a turbofan engine, in which the propulsive fan is driven by a corresponding turbine via a mechanical link there-between. However certain aircraft engine configurations over recent years have proposed the use of a distributed propulsion system, in which power generated by the engine core is used to drive a generator. The propulsive fan(s) can thus be driven by an electric motor, so as to allow greater flexibility in control between the engine(s) and fan(s).
Although the primary motivation for such alternative engine configurations is improved efficiency and hence reduced emissions of CO2, it has been found that the exhaust of an isolated engine core is prone to contrail formation over a potentially wider range of altitudes and/or atmospheric conditions than that of a conventional turbofan.
It is an aim of the present invention to provide a vapour trail mitigation system that specifically addresses a distributed aircraft propulsion configuration.